How Did Hamburgers Get So Darn Good?

The hamburger is at least a hundred years old, possibly much more depending on who you believe. It's a simple sandwich, a ground beef patty between two pieces of bread, plus a slice of cheese if you're lucky. It doesn't seem like a product that has room for much improvement, and yet, year after year, hamburgers keep on getting better. How is this possible? How can something with so little complexity continue to get tastier and tastier, year after year?